The present invention relates to submarine ploughs, and relates particularly, but not exclusively, to submarine ploughs for laying cables and other flexible products under the surface of the sea bed.
A substantial part of the world""s international telecommunications is transmitted by means of fibre optic cables laid on the sea bed. A single such cable can carry a very large number of simultaneous telephone calls, and any damage sustained by such cables can result in considerable financial loss. One of the main causes of damage to cables laid on the sea bed is fishing activities, and attempts are generally made to protect cables from such interference by burying them under the surface of the sea bed. The cables are generally buried by means of a submarine plough, which is towed behind a cable laying ship and picks up a cable, laid on the sea bed, into the plough and then buries it in a trench dug in the surface of the sea bed by the plough.
Hitherto, a burial depth of one meter or less has generally been regarded as sufficient, and towing the cable plough at this depth requires a pulling force of up to 50 tonnes.
A conventional plough for burying a cable to a depth of up to one meter in strong or hard soil is shown in FIG. 1 and comprises a plough share 1 which comprises an assembly of parts which cut and move the soil at the deepest part of a trench in the sea bed to bury a cable 2 which passes through the plough and is held down by a movable depressor 3. The plough share 1 is connected to a pair of depth control skids 4 at the front of the plough by means of a leg 17 and a long beam 5, the skids 4 being movable up or down relative to the beam 5 by moving skid support arms 6.
The cable plough runs at a generally constant depth by means of the long beam principle, which will be well known to persons skilled in the art. Under this principle, a cutting edge 7 of the plough share 1 cuts a flat bottom to the trench in the sealed, and a heel 8 supports the weight of the rear of the plough and slides along the soil surface cut by the cutting edge 7. Any tendency of the plough to alter the running depth, for example by means of the rear of the plough lifting up by pivoting about the front skids 4, is counteracted by the heel 8 lifting off the soil surface which in turn throws the weight of the rear of the plough on to the share 1 which is unable to support this additional load. As a result, the plough tends to run deeper, counteracting the movement of the rear of the plough. Conversely, it is difficult for the rear of the plough to go deeper because this requires the heel 8 to push down into the soil cut by the share cutting edge 7.
The plough is pulled by means of a tow rope 9 attached to a cable ship. The cable 2 enters the plough via a bellmouth device 10 designed to prevent the cable from being bent around radii which are too small (a typical minimum radius being of the order of 1.5 meters). If the cable 2 being laid is long it may have one or more joints or repeater amplifiers 11 at intervals along its length and which also have to pass through the plough and be buried with the cable 2. An auxiliary repeater burial share 12, which makes the cable trench wider, is used to provide the necessary space for the repeater units 11.
However, in weak soils, certain aggressive types of fishing gear can penetrate the sea bed to depth in excess of one meter, which exposes buried cables to the risk of damage. It is therefore desirable for cable ploughs to be able to bury cables deeper than one meter, for example up to three meters deep in weak soils such as soft muds. However, cable ploughs of this type, when operated in stronger soils, should also be capable of adjusting their ploughing depth to a shallower value to match the pulling force available from the cable laying ship.
FIG. 2, in which parts common to the embodiment of FIG. 1 are denoted by like reference numerals but increased by 100, denotes a conventional cable laying plough adapted to address this problem. The plough is intended to operate as deeply as possible using the 50 tonne pulling force required by a standard cable plough such that its maximum operating depth is not less than 3 meters in a weak soil such as soft mud. The plough of FIG. 2 is also intended to be not significantly larger or heavier than the existing plough of FIG. 1.
In effect, the plough of FIG. 2 consists of the plough of FIG. 1, modified to have a longer plough leg 117 and front skid arms 106 to enable the plough to reach greater depths. However, the plough of FIG. 2 has serious operational limitations which arise from the fact that the separation distance 113 between the horizontal components of the tow rope force 109 and the soil reaction 114 acting on the share 101 is very large. This is particularly so when the cable plough is digging a shallow trench in strong or hard soil, for example as shown in FIG. 2. The product of the forces 109, 114 with the separation distance 113 constitutes a couple which tends to lift the rear of the cable plough off the supporting trench surface, thus hampering penetration by the share 101. In addition, because the lower lip of bellmouth device 110 is high above the sealed, any slack in cable 102 can cause it to bend backwards on itself as shown at 115, which can cause the cable to wrap around the very small radius formed by the lower lip of the bellmouth 110. This can result in severe damage to the cable.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to the present invention, there is provided a submarine plough, the plough comprising:
a plough share having a cutting edge for cutting a bottom of a trench in a sea floor;
a heel portion arranged rearwardly of the cutting edge for engaging the bottom of the trench to at least partially support the plough;
a plough beam having an attachment portion for enabling towing means to be attached thereto to tow the plough to cut a trench in the sea floor, wherein the plough share is pivotable relative to the plough beam to adjust the vertical separation in use between the cutting edge and the attachment portion, sea floor engaging means arranged adjacent a forward end of the plough beam; and
adjustment means for pivoting the plough share relative to the plough beam;
wherein the plough is adapted to pass an elongate flexible product therethrough in an arcuate path into the trench rearwardly of the cutting edge and the heel portion.
By adjusting the working depth of the plough by pivoting the plough share relative to the plough beam, this provides the advantage of avoiding total reliance on adjustment of the height of the sea floor engaging means. This in turn enables the working depth of the plough to be adjusted without the necessity of increasing to unacceptable levels the height of the point of attachment of the towing means above the top of the trench. As result, the risk of the elongate flexible product bending backwards on itself, resulting in damage thereto, is minimised.
In one preferred embodiment, the plough beam is rigidly attached to a plough leg, the plough share is pivotably connected adjacent a rearward portion thereof to the plough leg, and the plough is adapted to receive an elongate flexible product adjacent a forward end of the plough beam and pass the product along a rear portion of the plough leg into the trench.
This provides the advantage of enabling the flexible product upstream of the trench to remain clear of the sea bed even when the plough is at its deepest working position.
The plough share may comprise a substantially part-cylindrical surface substantially coaxial with the pivot axis of said plough share about the plough leg, and adapted to slide against a cooperating substantially part-cylindrical surface on the plough leg.
This provides the advantage of minimising any gaps between the plough share and the plough leg, thus minimising the extent to which soil and other particulate material can penetrate the plough to cause jamming thereof.
The plough share preferably comprises a pair of substantially parallel arms extending rearwardly of the share and adapted to pivotably receive a cooperating portion of the plough leg therebetween.
The adjustment means may comprise one or more linear actuators.
Preferably, the or each said linear actuator comprises a hydraulic ram.
In preferred embodiment, the or each said linear actuator includes a respective cross head and a cross pin adapted to slide in one or more arcuate slots in the plough share leg, and the or each said cross head is adapted to tightly fit inside the plough share leg and to cover the corresponding slot at each position of the pin in the slot.
This reduces the extent to which sand and other particulate material can penetrate the arcuate slots, which may cause jamming of the plough.
The plough beam may comprise two beam members connected to said plough leg.
The plough share leg is preferably connected to the beam members by means of a pair of connecting members, at least one of which comprises a removable portion to enable an elongate flexible product to be mounted to the plough.
In another preferred embodiment, the plough share comprises an elongate plough leg to which said cutting edge and said heel portion are rigidly connected, and wherein the plough leg is pivotably connected adjacent an upper portion thereof to said plough beam.
By providing an elongate plough leg which is pivotable relative to the plough beam, this gives the advantage of enabling the pivot point to be kept clear of the bottom of the trench dug by the plough, thus minimising the risk of soil or other particulate material penetrating the moving parts of the plough.
The elongate flexible product may be a telecommunications cable.
The plough preferably further comprises a bellmouth device for receiving the elongate flexible product adjacent a forward end of the plough beam.
This provides the advantage of minimising the extent to which the flexible product is forced to bend around small radii of curvature which, in the case of a cable, may otherwise cause damage.
In a preferred embodiment, the sea floor engaging means comprises one or more skids connected to the plough beam by means of a respective adjustable arm.
The plough may further comprise additional sea floor engaging means arranged rearwardly of the sea floor engaging means.
In a preferred embodiment, said additional sea floor engaging means comprises one or more skids connected to a rearward portion of the plough beam.
This provides the advantage of providing additional support to prevent the plough from sinking into soft mud.
The adjustment means may further comprise indicator means for indicating the orientation of the plough share relative to the plough beam.
The plough preferably further comprises a pair of flaps located rearwardly of the plough beam, said flaps being displaceable to allow the passage through the plough of wider portions of the elongate flexible product.
In the case of the flexible product being a telecommunications cable, this provides the advantage of enabling repeater units on the cable to be accommodated in the trench.